This invention relates generally to document illumination systems and more particularly to document illumination systems which are adapted for use in a high speed jet drop copier. A typical prior art illumination system for a jet drop copier is disclosed in Paranjpe et al U.S. Pat. No. 4,112,469. The system as disclosed comprises a pair of stationary illumination lamps and reflectors.
An illumination arrangement as disclosed in Paranjpe et al provides only a relatively low level of illumination across the face of the document being copied. This relatively low level of incident light is reflected through a set of imaging optics for focussing against the face of a photodetector array. The photodetectors comprising the photodetector array generate jet drop printing control signals in response to the light reflected from the document.
The signals generated by the photodetectors in response to light received from the document must compete with noise signals which are inherently generated by the system electronics. When the light levels are quite low, the photodetectors must have a relatively long observation time in order to generate reliable printing control signals. Thus as the copying speed increases, there is a requirement for a concomitant increase in the level of illuminating light. In the specific jet drop copying system for which the present invention is intended, document scanning is carried out at a linear speed in the order of about 128 in. per second, and the scanning resolution is about 535 lines per inch. This means that the photodetector observation time is only about 14.6 microseconds. In order for such a system to tolerate normal optical system losses and to achieve a photodetector signal-to-noise ratio of 20 db, an illumination level of about 0.38 watts per cm.sup.2 is required. This illumination level is much higher than that which is provided by conventional document illumination systems.